1. Technical Field
The present invention relates generally to a voice over Internet protocol (VoIP) system and, in particular, to a routing service method in a VoIP system using a stand-alone gateway.
2. Related Art
VoIP is a novel communication service that delivers voice information over the Internet rather than over the public switched telephone network (PSTN). A major advantage of VoIP telephony is that it avoids the tolls charged by ordinary telephone service, including domestic/international calls, because VoIP is implemented in a packet-based network. VoIP sends video information, as well as audio information, using International Telecommunications Union-Telecommunications standardization sector (ITU-T) H.323.
A VoIP system includes the Internet. The Internet is connected to gateways and personal computers (PCs). The gateways are connected to respective PSTNs that are in turn connected to telephones. The telephones and the PCs are endpoints that can communicate in voice (mandatory), with pictures (optional), and with data (optional) in one-to-one conversations or conferences. These terminals can carry out real-time bi-directional communications with the gateways or other terminals. The gateways enable terminals (e.g., the PCs) connected to the Internet, which is a packet-based network, or terminals (e.g., the telephones) connected to the PSTNs or to the integrated services digital network (ISDN), to make real-time bi-directional communications. For example, the gateways send voice and fax data received from the PSTNs to the Internet after real-time compression and protocol conversion.
Such gateways are usually categorized into three types according to installation and usage: a built-in type that is inserted in the form of a card in a key telephone system (KTS) or a private branch exchange (PBX); a server type installed in a platform like window network terminal (NT); and a stand-alone type independent of other terminals. Stand-alone gateways are divided into trunk and ear and mouth (TANDEM) types and stand-alone types according to their functionalities. A stand-alone gateway with the TANDEM functionality supports interworking between disparate central office (CO) lines. The TANDEM stand-alone gateway is connected to a PBX and/or a KTS via an internal T1/E1 interface, a loop start trunk interface, and a subscriber line circuit (SLC) interface. A stand-alone gateway with the stand-alone functionality is connected directly to a plurality of telephones.
As to a routing service, when attempting a VoIP call in a VoIP system using stand-alone gateways upon origination of a VoIP call from a caller, the VoIP call is transferred to a local stand-alone gateway via a calling PBX. The local stand-alone gateway routes the VoIP call to a called party via a route which is chosen as the best one. The route runs from the caller to the called party through the calling PBX, the local stand-alone gateway, the Internet, a remote stand-alone gateway, and a called PBX.
If the local stand-alone gateway senses that the route is not available for the call service due to, for example, link-down of the remote stand-alone gateway, it chooses the second best route. The route runs from the caller to the called party through the calling PBX, the local stand-alone gateway, a PSTN central office (CO) line, a PSTN, a PSTN, and the called PBX. If the best route is not available, the local stand-alone gateway takes a roundabout route through the PSTNs in order to route the call to the called party. In order to take the roundabout route via the CO line, the local stand-alone gateway and the remote stand-alone gateway should have CO line interfaces.
To offer such a roundabout route call service over a legacy network, the CO line should be prepared just in case. Preservation of the CO line for access to a legacy network, such as the PSTN, is a redundancy from the VoIP system's perspective. As a result, the line interfaces of the stand-alone VoIP system are or become less flexible.